A distributed grid-connected inverter system is used for connecting a distributed power supply to a grid through grid-connected inverters. In order to reduce the influence of harmonics output by the grid-connected inverters on the grid, many methods are proposed in the existing patents, but the research objects of these methods are single inverters, which can only reduce the harmonics of one inverter. Under normal circumstances, the grid-connected inverters are distributed at different positions, as shown in FIG. 1, each grid-connected inverter is independently controlled by a controller thereof to send or absorb power from the grid. The quality of output waveforms of the inverters can be controlled by a pulse width modulation method. Although each inverter controls the total harmonic distortion rate of the output current thereof within an allowable range to meet the requirements of the national standard, when a lot of inverters are connected to a point of common coupling or a local grid, since the pulse width modulation waves of the inverters cannot satisfy the phase relationship of mutually counteracting the harmonic current between the inverters, the total current harmonic of the grid is in a disordered superimposition state, the superimposition result of the harmonics of the grid is unknown, and may be more serious, or there may be partial counteraction of each other.
As shown in FIG. 2, for convenience of expression, the inverter is split into two parts, namely a controller and a power circuit in FIG. 2, and the distributed power supply is omitted. The total harmonic distortion of the output current of each inverter is smaller than a certain determined value, namely THDm<Xm % (m=1, 2 . . . N), the inverters are independently controlled by the controllers, the pulse width modulation waves of the inverters cannot satisfy the phase relationship of mutually counteracting the harmonic current between the inverters, namely t(m)−t(1)=? in FIG. 2 is an indeterminate value, the disorderly superposed current harmonics can make the total harmonic distortion rate of the current of the grid exceed the requirements of the grid, that is, THD<? in FIG. 2 expresses that the total harmonic distortion is indeterminate, or when the line impedance is relatively large, voltage distortion of the point of common coupling can be caused to bring a serious power quality problem. In the existing patents, only the current harmonics output by one inverter can be reduced, while the problem of the current harmonics injected by the point of common coupling into the grid cannot be controlled, some articles mention the synchronization of different inverters, but are aimed at the synchronization on power frequency of parallel operation inverters at the same location, but mention no harmonic elimination problem. The patent proposes a global synchronous pulse width modulation method, in which global pulse width modulation is performed on inverters distributed at different locations to effectively reduce the current harmonics injected by the point of common coupling into the grid.